Apparatus for forming magnetite electrostatographic carriers

ABSTRACT

In preferred form, the inventive concept utilizes a carbon arc plasma flame assembly wherein magnetic ore particles are fed to the plasma flame assembly in the presence of a controlled, gaseous atmosphere. The plasma flame assembly comprises a cathode and at least two carbon anodes to which is fed oxygen gas in a controlled amount. Upon contact with the plasma flame, the ore particles become molten droplets which fall by gravity into a spheroidization column having a controlled atmosphere. As the molten droplets fall, they cool and become spheroidal particles. The chamber wherein the heating occurs is substantially sealed and is initially purged with inert gas. In addition, the spheroidization column is fitted with an exhaust system which maintains the integrity of the processing atmosphere and that of the spheroidized particles.

BACKGROUND OF THE INVENTION

This application is a divisional application of copending applicationSer. No. 552,589, filed on Feb. 24, 1975.

This invention relates in general to electrostatographic imagingsystems, and in particular, to apparatus for preparing improvedmagnetically attractable developer materials.

Electrostatographic imaging processes and techniques have beenextensively described in both the patent and other literature, forexample, U.S. Pat. Nos. 2,221,776, 2,277,013, 2,297,691, 2,357,809,2,551,582, 2,285,814, 2,833,648, 3,220,324, 3,220,831, 3,220,833, andmany others. Generally, these processes have in common the steps ofemploying a normally insulating photoconductive element which isprepared to respond to imagewise exposure with electromagnetic radiationby forming a latent electrostatic charge image. The electrostatic latentimage is then rendered visible by a development step in which thecharged surface of the photoconductive element is brought into contactwith a suitable developer mix.

One method for applying the developer mix is by the well-known magneticbrush process. Such a process can utilize apparatus of the typedescribed, for example, in U.S. Pat. No. 2,874,063, and often comprisesa non-magnetic rotatably mounted cylinder having fixed magnetic meansmounted inside. The cylinder is arranged to rotate so that part of thesurface is immersed in or otherwise contacted with a supply of developermix. The granular mass comprising the developer mix is magneticallyattracted to the surface of the cylinder. As the developer mix comeswithin the influence of the field generated by the magnetic means withinthe cylinder, the particles thereof arrange themselves in bristle-likeformations resembling a brush. The bristle formations of developer mixtend to conform to the lines of magnetic flux, standing erect in thevicinity of the poles and lying substantially flat when said mix isoutside the environment of the magnetic poles. Within one revolution thecontinually rotating tube picks up developer mix from a supply sourceand returns part or all of this material to the supply. This mode ofoperation assures that fresh mix is always available to the copy sheetsurface at its point of contact with the brush. In a typical rotationalcycle, the roller performs the successive steps of developer-mix pickup,brush formation, brush contact with the photoconductive element, brushcollapse and finally mix release.

In magnetic-brush development of electrostatic images, the developer iscommonly a triboelectric mixture of fine toner powder comprised of dyedor pigmented thermoplastic resin with coarser carrier particles of asoft magnetic material such as "ground chemical iron" (iron filings),reduced oxide particles, or the like. The conductivity of theferromagnetic carrier particles which form the "bristles" of a magneticbrush gives some advantage over other modes of development. Theconductivity of the ferromagnetic fibers or bristles provides the effectof a development electrode having a very close spacing to the surface ofthe electrophotographic element being developed. By virtue of thisdevelopment electrode effect, it is to some extent possible to developpart of the tones in pictures and solid blacks as well as line copy.This ability to obtain solid area development with magnetic brushdeveloping makes this mode of developing advantageous where it isdesired to copy materials other than simple line copy.

However, most currently available ferromagnetic carrier particles havean electrical resistance which is too high to produce good quality solidarc development. The various commercial carrier particles generally lackadequate conductivity because of the presence of an insulating surfacelayer of iron oxide, grease or other contaminants. Efforts to removesuch surface contaminants often result in particles which have an evenhigher electrical resistivity. For example, washing or solvent treatmentof iron carrier particles in an effort to remove contaminants merelyexposes the surface of the underlying iron to aerial oxidation. The newlayer of oxide often has far greater resistivity than the originalcontaminants. Such an oxide coating can be removed; however, specialafter-treatment and precaution in storage and handling are required inorder to avoid any further oxidation.

Electrostatographic carrier surfaces and carrier particles are generallymade from or coated with materials having appropriate triboelectricproperties as well as certain other physical characteristics. However,the carrier substrate as well as the surface thereof should not becomprised of materials which are so brittle as to cause either flakingof the surface or particle breakup under the forces exerted on thecarrier during recycle. The flaking thereof causes undesirable effectsin that the relatively small flaked particles will eventually betransferred to the copy surface thereby interfering with the depositedtoner and causing imperfections in the copy image. Furthermore, flakingof the carrier surface will cause the resultant carrier to havenon-uniform triboelectric properties when the carrier is composed of amaterial different from the surface coating thereon. This results inundesirable non-uniform pickup of toner by the carrier and non-uniformdeposition of toner on the image. In addition, when the carrier particlesize is reduced, the removal of the resultant small particles from thephotoconductive plate becomes increasingly difficult. Thus, the types ofmaterials useful for making carrier or for coating carrier, althoughhaving the appropriate triboelectric properties, are limited becauseother physical properties which they possess may cause the undesirableresults discussed above.

Thus, there is a continuing need for a better developer material fordeveloping electrostatic latent images.

SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to provide apparatus forpreparing carrier materials which overcome the abovenoted deficiencies.

It is another object of this invention to provide apparatus formanufacturing electrostatographic carrier particles and the resultingproducts which overcome the above-noted deficiencies.

It is another object of this invention to provide apparatus formanufacturing electrostatographic carrier particles having improvedmagnetic and electrical properties.

It is another object of this invention to provide apparatus formanufacturing electrostatographic carrier particles having improvedtriboelectric properties.

It is a still further object of this invention to render suitable aselectrostatographic carrier particles materials which were heretoforeunsuitable as carrier materials.

It is a still further object of this invention to provide apparatus forpreparing improved carrier materials having physical and chemicalproperties superior to those of known developer materials.

The above objects and others are accomplished, generally speaking, byintroducing low cost magnetic ore particles to a plasma flame heatingdevice in the presence of at least one gas, melting and spheroidizingthe ore particles in a closed chamber having a controlled atmospherewherein the ore droplets are allowed to fall by gravity and cool, andcollecting the droplets. More particularly, the apparatus of thisinvention is employed by placing magnetic ore particles having anaverage particle diameter from between about 5 microns and about 600microns in a powder feeder, feeding the ore particles to a plasma flamein the presence of at least one gas having good heat transfer propertieswhere the ore particles are melted, allowing molten droplets of the oreparticles to fall by gravity in a substantially closed chamber having acontrolled gaseous atmosphere wherein the molten droplets spheroidizeand cool, and collecting the spheroid solid particles.

In a preferred form, the inventive concept utilizes a carbon arc plasmaflame furnace having a 400 kw arc head with one cathode and three carbonanodes whereby temperatures of up to about 10,000° K. are attained. Theplasma flame furnace is positioned atop a jacketed chamber having adiameter of about 36 inches and a height of about 22 feet. The highgravity drop assures solidification of each spheroidized particle. Theregion where the carbon arc plasma flame is formed between the cathodeand the anodes is termed the arc region and provides a flame operatingin a downward direction. The chamber wherein the heating process occursis substantially sealed and is initially purged with inert gas. Inaddition, the plasma flame assembly including the cathode head and thespheroidization chamber are cooled with water jackets.

The foregoing objects, brief description, advantages, and features ofthe present invention will become more apparent from the following moredetailed description and appended drawings wherein:

FIG. 1 is a generalized schematic diagram of the carbon arc plasma flameassembly and the spheroidization column; and,

FIG. 2 illustrates the internal operation of the carbon arc plasma flameassembly.

Referring to the general assembly schematic in FIG. 1, thespheroidization column 10 is fitted with a carbon arc plasma flameassembly 12. The spheroidization column and more specifically the plasmaflame assembly is provided with powder feeder 14 including feeder tubes16 having feed ports for directing the ore to the arc region. Thespheroidization column is typically from between about 10 feet and about30 feet in height and is fitted with a water jacket 18 and an exhaustoutlet 20 leading to a dry cyclone collection chamber 22 fitted with bagfilters 24. In addition, the spheroidization column is connected with apurge system indicated as inlet conduit 28 providing an internal flow ofnitrogen to the spheroidization column. The purged effluent exits thespheroidization column via exhaust outlet 20. After spheroidization andcooling of the ore particles inside the spheroidization column, theresultant product is recovered as beads via the output conduit 30 fromthe lower portion of the spheroidization column. The beads thus providedmay be discharged into a drying unit 32 when the molten droplets arecooled with a water bath at the bottom of the spheroidization chamber. Ascreening unit 34 may serve the functions of classifying particles inaccordance with a desired mesh size, and placing the desired particlesin the receptacle 36 and the undesired particles in the scrap unit 38.

Referring now to FIG. 2, the interior of the carbon arc plasma flameassembly is shown in greater detail. The ore particles are introduced tothe plasma flame assembly via feeder ports 17 from ore feeder tubes 16along with an inert non-oxidizing carrier gas such as nitrogen by meansof a high pressure supply or the like, shown in FIG. 1. Cathode 19 isconnected to a DC power supply system, not shown. In addition, Cathode19 is provided with a source of argon gas fed through lines 21 by meansof a high pressure supply or the like, not shown. Carbon anodes 23 aremounted above the insulating upper surface of the spheroidization columnand are positioned perpendicular to the cathode. The anodes are fed,along with nitrogen gas which acts as a seal at the anode port to thearc region as to maintain the desired electrical potential between eachanode and the cathode. Cathode 19 in cooperation with carbon anodes 23provide the electrical arcs which ionize the feed gases to form plasmaflame 25. A controlled amount of oxygen gas is fed to plasma flame 25via port 27.

In operation, powder feeder 14 is loaded with magnetic ore particles ofthe approximate desired particle size. Cathode 19 and carbon anodes 23are energized by an electrical power source and in cooperation with thefeed gases provide plasma flame 25. The ore particles are fed via feedertubes 16 to the carbon arc plasma flame in the presence of an inertnon-oxidizing carrier gas. It has been found that feeding the oreparticles in the presence of nitrogen gas provides good heat transfer tothe ore particles during processing thereby improving the efficiency ofthe process. Other gases which have good heat transfer properties mayalso be used for this purpose.

Due to the extreme heat generated in the plasma arc region, the oreparticles passing therethrough become melted globules. As the meltedglobules fall by gravity into the spheroidization chamber, the globulesspheroidize, cool, and collect at the bottom of the spheroidizationchamber. The proper conditions for the plasma flame are created byadjusting the flow rate of the ore particles, the flow rate of the feedgases, and the voltage and current of the cathode and of the anodes.

The plurality of particles thus formed, indicated generally as 44, fallaway from the arc region of the plasma flame as droplets and down to thebottom of the spheroidization chamber to collect near output conduit 30.As they fall, the droplets are cooled and solidify in spherical form.The collecting area serves also to cool as well as collect thespheroidized particles. From the collecting area, the particles aredischarged through output conduit 30 by gravity, an auger feed, or othersuitable means for removal. The particles are introduced to screeningunit 34 for classification and ultimate use as electrostatographiccarrier particles.

To further improve the characteristics of the spheroid particles thusformed, an exhaust system is utilized as indicated at outlet 20. Theexhaust system has been found beneficial in removing dust and fines fromthe spheroidization column and avoids their settling on and otherwisecontaminating the spheroidized particles. The dust and fines therebyremoved from the spheroidization chamber are passed through cyclonecollection chamber 22 and collected for removal by bag filters 24.Element 20 is provided with a suitable means (now shown) such as a backpressure valve or the like which will allow release of gases whilemaintaining a sufficiently high positive pressure in the columnresulting from the continuous feeding of the gases into the chamber. Inthis manner the spheroidization column may continuously be ventedwithout the fear of backwash resulting in contamination from air or thelike.

The spheroidization column is of sufficient height and diameter topermit the molten droplets to assume their spheroidal shape beforestriking the output conduit. It has been found experimentally that goodresults are obtained in a column having a 36 inch diameter and a 22 footheight, However, it has been found that satisfactory results withvarying yields may be obtained from a height ranging from about 5 toabout 10 feet, and it is estimated that a diameter of about 2 feet orgreater will suffice.

It is preferred that the spheroidization column wall be jacketed withsuitable interwall circulation of coolants or the like to providecooling for the chamber walls, the molten droplets, and to providefurther insurance against agglomeration of the spheroid particles.

The factors generally affecting the process yield and product qualityinclude, in order of relative importance, the amount of the oxidizinggas, the amount of inert, non-oxidizing gas, current flow, ore particletrajectory and feed rate, and the exhaust system. As noted above, theinert gas should be non-oxidizing or at least substantiallynon-oxidizing and should be inert with respect to the materials utilizedto form the particles. It has been found that nitrogen is a suitable gasfor this purpose, and is preferred. However, it is also possible toemploy argon, helium, carbon dioxide, or combinations of these or otherinert, non-oxidizing gas. It has further been found that the presence ofan oxidizing gas, with oxygen being preferred, is necessary at theplasma flame to obtain spheroidized particles having the desiredmagnetic moment for use as magnetic electrostatographic carrier beads.Thus, the presence of between about 10 percent and about 20 percent ofoxygen at the plasma flame has been found to provide the aforementioneddesired property to the beads. It has been found that magnetic oreparticles processed in accordance with this invention will providecarrier beads having an average saturation magnetic moment of betweenabout 50 and about 85 electromagnetic units per gram which results insatisfactory performance when they are used in magneticelectrostatographic development systems. However, carrier beads havingan average saturation magnetic moment of at least about 70electromagnetic units per gram are preferred because carrier beadshaving low magnetic moments have been found to result in carrier beadcarry-over to the recording surfaces. As more fully shown in the workingexamples, the magnetic moment of magnetic ore particles may be varieddepending upon the process conditions, and in particular, upon theamount of oxidizing gas present.

It is also highly desirable to reduce the formation of dust, e.g.,particles having an average diameter of less than about 2 microns. Thismay be accomplished by initially employing larger ore particle sizes andby preventing the introduction of "fines" or very small particles intothe plasma flame.

In the spheroidization process, employing a nominal 400 kw plasma arcunit, it has been found that acceptable results providing yieldssignificantly higher than obtainable with prior art processes, areachieved with an inert gas flow rate ranging between about 700 and about800 standard cubic feet per hour, an oxidizing gas flow rate rangingbetween about 100 and about 200 standard cubic feet per hour, and anelectrical current flow of between about 650 and about 750 amperesbetween each anode and the cathode. The ore particle feed rate may varybetween about 100 to about 400 pounds per hour.

Various sources of magnetic ore particles may be employed to produceother metallic or pseudo-alloy core materials. Typical ore sourcesinclude magnetite, hematite, taconite, ilmenite, and the like. Inaddition, it is preferred to screen the raw ore particles prior to theirprocessing in accordance with this invention. Screening orclassification of the raw ore particles will provide ore material havingbetter controlled particle sizes wherein after processing, the beadswill be of more uniform size. In addition, where magnetic beads aredesired, it is advantageous to magnetically separate the magnetic orefrom non-magnetic fractions prior to processing.

The low cost magnetic ore carrier materials processed in accordance withthis invention may be coated with any suitable coating material. Typicalelectrostatographic carrier particle coating materials include vinylchloride-vinyl acetate copolymers, poly-p-xylene polymers,styrene-acrylate-organosilicon terpolymers, natural resins such ascaoutchouc, colophony, copal, dammar, Dragon's Blood, jalap, storax;thermoplastic resins including the polyolefins such as polyethylene,polypropylene, chlorinated polyethylene, and chlorosulfonatedpolyethylene; polyvinyls and polyvinylidenes such as polystyrene,polymethylstyrene, polymethyl methacrylate, polyacrylonitrile, polyvinylacetate, polyvinyl alcohol, polyvinyl butyral, polyvinyl chloride,polyvinyl carbazole, polyvinyl ethers, and polyvinyl ketones;fluorocarbons such as polytetrafluoroethylene, polyvinyl fluoride,polyvinylidene fluoride; and polychlorotrifluoroethylene; polyamidessuch as polycaprolactam and polyhexamethylene adipamide; polyesters suchas polyethylene terephthalate; polyurethanes; polysulfides,polycarbonates; thermosetting resins including phenolic resins such asphenol-formaldehyde, phenol-furfural and resorcinol formaldehyde; aminoresins such as urea-formaldehyde and melamineformaldehyde; polyesterresins; epoxy resins; and the like.

Many of the foregoing and other typical carrier coating materials aredescribed by L. E. Walkup in U.S. Pat. No. 2,618,551; B. B. Jacknow etal U.S. Pat. No. 3,526,533; and R. J. Hagenbach et al in U.S. Pat. Nos.3,533,835 and 3,658,500. When the magnetic carrier particles of thisinvention are coated, any suitable electrostatographic carrier coatingthickness may be employed. However, a carrier coating having a thicknessat least sufficient to form a thin continuous film on the carrierparticle is preferred because the carrier coating will then possesssufficient thickness to resist abrasion and prevent pinholes whichadversely affect the triboelectric properties of the coated carrierparticles. Generally, for cascade and magnetic brush development, thecarrier coating may comprise from about 0.1 percent to about 10.0percent by weight based on the weight of the coated composite carrierparticles. Preferably, the carrier coating should comprise from about0.1 percent to about 1.0 percent by weight based on the weight of thecoated carrier particles because maximum durability, toner impactionresistance, and copy quality are achieved. To achieve further variationin the properties of the coated magnetic carrier particles, well-knownadditives such as plasticizers, reactive and non-reactive polymers,dyes, pigments, wetting agents and mixtures thereof may be mixed withthe coating materials. An ultimate coated or uncoated carrier particlehaving an average diameter between about 30 microns and about 600microns is preferred in development systems because the carrier particlethen possesses sufficient density and inertia to avoid adherence to theelectrostatic image during the development process. Adherence of carrierparticles to an electrostatographic drum is undesirable because of theformation of deep scratches on the drum surface during the imagetransfer and drum cleaning steps, particularly where cleaning isaccomplished by a web cleaner such as the web disclosed by W. P. Graff,Jr., et al in U.S. Pat. No. 3,186,838.

Any suitable well-known toner material may be employed with the magneticcarriers of this invention. Typical toner materials include gum copal,gum sandarac, rosin, cumaroneindene resin, asphaltum, gilsonite,phenolformaldehyde resins, rosin modified phenolformaldehyde resins,methacrylic resins, polystyrene resins, polypropylene resins, epoxyresins, polyethylene resins, polyester resins, and mixtures thereof. Theparticular toner material to be employed obviously depends upon theseparation of the toner particles from the magnetic carrier in thetriboelectric series and should be sufficient to cause the tonerparticles to electrostatically cling to the carrier surface. Among thepatents describing electroscopic toner compositions are U.S. Pat. No.2,659,670 to Copley; U.S. Pat. No. 2,753,308 to Landrigan; U.S. Pat. No.3,079,342 to Insalaco; U.S. Pat. No. Re. 25,136 to Carlson and U.S. Pat.No. 2,788,288 to Rheinfrank et al. These toners generally have anaverage particle diameter between about 1 and 30 microns.

Any suitable colorant such as a pigment or dye may be employed to colorthe toner particles. Toner colorants are well known and include, forexample, carbon black, nigrosine dye, aniline blue, Calco Oil Blue,chrome yellow, ultramarine blue, Quinoline Yellow, methylene bluechloride, Monastral Blue, Malachite Green Ozalate, lampblack, RoseBengal, Monastral Red, Sudan Black BM, and mixtures thereof. The pigmentor dye should be present in the toner in a quantity sufficient to renderit highly colored so that it will form a clearly visible image on arecording member. Preferably, the pigment is employed in an amount fromabout 3 percent to about 20 percent, by weight, based on the totalweight of the colored toner because high quality images are obtained. Ifthe toner colorant employed is a dye, substantially smaller tonerconcentration may be employed with the magnetic carriers of thisinvention. Typical toner concentrations for electrostatographicdevelopment systems include about 1 part toner with about 10 to about200 parts by weight of carrier.

Any suitable organic or inorganic photoconductive material may beemployed as the recording surface with the magnetic carriers of thisinvention. Typical inorganic photoconductor materials include: sulfur,selenium, zinc sulfide, zinc oxide, zinc cadmium sulfide, zinc magnesiumoxide, cadmium selenide, zinc silicate, calcium strontium sulfide,cadmium sulfide, mercuric iodide, mercuric oxide, mercuric sulfide,indium trisulfide, gallium selenide, arsenic disulfide, arsenictrisulfide, arsenic triselenide, antimony trisulfide, cadmiumsulfo-selenide, and mixtures thereof. Typical organic photoconductorsinclude: quinacridone pigments, phthalocyanine pigments, triphenylamine,2,4-bis(4,4'-diethyl-amino-phenol)-1,3,4-oxadiazol,N-isopropyl-carbazol, triphenyl-pyrrol, 4,5-diphenylimidazolidinone,4,5-diphenylimidazolidinethione,4,5-bis-(4'-amino-phenyl)-imidazolidinone, 1,5-dicyanonaphthalene,1,4-dicyanonaphthalene, aminophthalodinitrile, nitrophthalodinitrile,1,2,5,6-tetraazacyclooctatetraene-(2,4,6,8),2-mercaptobenzothiazole-2-phenyl-4-diphenylideneoxazolone,6-hydroxy-2,3-di(p-methoxy-phenyl)-benzofurane,4-dimethylamino-benzylidene-benzhydrazide,3-benzylidene-amino-carbazole, polyvinyl carbazole,(2-nitro-benzylidene)-p-bromoaniline, 2,4-diphenyl-quinazoline,1,2,4-triazine, 1,5-diphenyl-3-methyl-pyrazoline, 2-(4'-dimethylaminophenyl)-benzoxazole, 3-amine-carbazole, and mixtures thereof.Representative patents in which photoconductive materials are disclosedinclude U.S. Pat. No. 2,803,542 to Ullrich, U.S. Pat. No. 2,970,906 toBixby, U.S. Pat. No. 3,121,006 to Middleton, U.S. Pat. No. 3,121,007 toMiddleton, and U.S. Pat. No. 3,151,982 to Corrsin.

The suprisingly better results obtained with the electrostatographic lowcost magnetic carriers of this invention may be due to many factors. Forexample, the spheroidized carriers of this invention possess smoothouter surfaces which are highly resistant to cracking, chipping, andflaking. In electrostatographic development systems, the sphericalsurface enhances the triboelectric action of the carrier particlesacross the electrostatographic surfaces and reduces the tendency ofcarrier particles to adhere to electrostatographic imaging surfaces.When these carriers are employed in electrostatographic developmentsystems, carrier life is unexpectedly extended particularly with respectto toner impaction resistance. Additionally, the carriers of thisinvention provide more uniform triboelectric characteristics thancurrent carriers when employed in electrostatographic developmentsystems. Further, the carriers of this invention provide exceptionallygood life performance, durability, copy quality, quality maintenance,less carrier bead sticking and agglomeration, and also provideeconomical carrier materials thereby minimizing the cost ofelectrostatographic developer materials. Thus, the magnetic carrierparticles of this invention have desirable properties which permit theirwide use in presently available electrostatographic systems.

The following examples, other than the control examples, further define,describe and compare preferred methods of preparing and utilizing themagnetic carriers of the present invention in electrostatographicapplications. Parts and percentages are by weight unless otherwiseindicated.

EXAMPLE 1

Employing the apparatus described in FIG. 1 and FIG. 2, nominal 100micron magnetite ore particles were placed in the powder feeder. The oreparticles were fed to the plasma flame at the rate of about 300pounds/hour. Nitrogen gas was fed with the ore particles at the rate ofabout 230 standard cubic feet per hour. In addition, argon gas was fedaround the cathode at the rate of about 230 standard cubic feet perhour. The carbon arc plasma flame furnace arc head was provided withabout 215 kw providing about 650 amperes between each of the threecarbon anodes and the cathode.

As the ore particles were injected into the plasma flame they becamemolten droplets. As the molten droplets fell by gravity in thespheroidization column, they cooled and spheroidized. The inside chamberof the spheroidization column was provided with a controlled atmospherehaving been fed nitrogen gas at the rate of about 290 standard cubicfeet per hour via the anode ports. After processing of the particles,the beads were collected from the bottom of the chamber via outputconduit 30 and classified.

Processed beads classified to 100 micron nominal particle size were thencoated with about 0.63 percent by weight, based on the weight of thebeads, of a coating material comprising sytrene-methacrylateester-organosilicon terpolymer as described in U.S. Pat. No. 3,467,634.A developer mixture was prepared comprising about 99 parts of the thuscoated beads and about 1 part of toner particles as described in U.S.Pat. No. 3,079,342. The developer mixture was employed in a magneticbrush electrostatographic system for the development of electrostaticlatent images. However, it was found that the developer mixture wasunsatisfactory in that it did not form a magnetic brush. The problem wasfound to be due to the very low magnetic moment of the carrier beadswhich was only about 21 electromagnetic units per gram.

EXAMPLE II

Employing the apparatus described in FIG. 1 and FIG. 2, nominal 100micron magnetite ore particles were placed in the powder feeder. The oreparticles were fed to the plasma flame at the rate of about 300pounds/hour. Nitrogen gas was fed with the ore particles at the rate ofabout 230 standard cubic feet per hour. Oxygen gas was fed to the plasmaat the rate of about 140 standard cubic feet per hour. In addition,argon gas was fed around the cathode at the rate of about 230 standardcubic feet per hour. The presence of argon gas around the cathode hasbeen generally found to improve its useful lifetime. The carbon arcplasma flame furnace arc head was provided with about 245 kw providingabout 680 amperes between each of the three carbon anodes and thecathode.

As the ore particles were injected into the plasma flame they becamemolten droplets. As the molten droplets fell by gravity in thespheroidization column, they cooled and spheroidized. The inside chamberof the spheroidization column was provided with a controlled atmospherehaving been fed with about 290 standard cubic feet per hour of nitrogengas via the anode ports. After processing of the ore particles, thebeads were then collected from the bottom of the chamber via outputconduit 30 and classified.

Processed beads classified to 100 micron nominal particle size were thencoated with about 0.62 percent by weight, based on the weight of thebeads, of a coating material comprising styrene-methacrylateester-organosilicon terpolymer as described in U.S. Pat. No. 3,467,634.A developer mixture was prepared comprising about 99 parts of the thuscoated beads and about 1 part of toner particles as described in U.S.Pat. No. 3,079,342. The developer mixture was employed in a magneticbrush electrostatographic system for the development of electrostaticlatent images. It was found that the developer mixture formed anexcellent magnetic brush and provided satisfactory reproductions oflatent images with high image densities and low background levels up toabout 200,000 copies. The magnetic moment of the carrier beads was foundto be about 72 electromagnetic units per gram. In addition, the uncoatedcarrier beads were found to have a substantially uniform densityindicating reliability of the process and low oxide contamination.

EXAMPLE III

Employing the apparatus described in FIG. 1 and FIG. 2, nominal 100micron magnetic ore particles were placed in the powder feeder. The oreparticles were fed to the plasma flame at the rate of about 300pounds/hour. Nitrogen gas was fed with the ore particles at the rate ofabout 230 standard cubic feet per hour. Oxygen gas was fed to the plasmaflame at the rate of about 80 standard cubic feet per hour. In addition,argon gas was fed around the cathode at the rate of about 230 standardcubic feet per hour. The carbon arc plasma flame furnace arc head wasprovided with about 245 kw providing about 600 amperes between each ofthe three carbon anodes and the cathode.

As the ore particles were injected into the plasma flame they becamemolten droplets. As the molten droplets fell by gravity in thespheroidization column, they cooled and spheroidized. The inside chamberof the spheroidization column was provided with a controlled atmospherehaving been fed with about 290 standard cubic feet per hour of nitrogengas via the anode ports. After processing of the ore particles, thebeads were then collected from the bottom of the chamber via outputconduit 30 and classified.

Processed beads classified to 100 micron nominal particle size were thencoated with about 0.62 percent by weight, based on the weight of thebeads, of a coating material comprising styrene-methacrylateester-organosilicon terpolymer as described in U.S. Pat. No. 3,467,634.A developer mixture was prepared comprising about 99 parts of the thuscoated beads and about 1 part of toner particles as described in U.S.Pat. No. 3,079,342. The developer mixture was employed in a magneticbrush electrostatographic system for the development of electrostaticlatent images. It was found that the developer mixture formed anexcellent magnetic brush and provided satisfactory reproductions oflatent images up to about 260,000 copies. The magnetic moment of thecarrier beads was found to be about 57 electromagnetic units per gram.

The particular configuration of the carbon arc plasma flame assembly isintended as exemplary only and is not intended to be limiting.

While the invention has been described with reference to specificpreferred embodiments, it will be apparent to those skilled in the artthat various substitutions, alterations and modifications may be madetherein without departing from the spirit and scope of the invention.Such substitutions, alterations and modifications are intended to bewithin the scope of this invention.

What is claimed is:
 1. An apparatus for manufacturing spheroidized beadscomprising a walled chamber, a carbon arc plasma flame assemblypositioned atop said walled chamber, a vibrating feeder means forfeeding ore particles in the presence of an inert gas via feed ports tosaid plasma flame assembly, means for feeding at least one gas to saidplasma flame assembly, a purge system connected to the upper portion ofsaid walled chamber, an exhaust system leading from the lower portion ofsaid walled chamber, and an output conduit located at the bottom of saidwalled chamber for recovery of said spheroidized beads.
 2. An apparatusfor manufacturing spheroidized beads in accordance with claim 1 whereinsaid inert gas is nitrogen.
 3. An apparatus for manufacturingspheroidized beads in accordance with claim 1 wherein said carbon arcplasma flame assembly is connected to a DC power supply.
 4. An apparatusfor manufacturing spheroidized beads in accordance with claim 1 whereinsaid carbon arc plasma flame assembly comprises a cathode and threecarbon anodes.
 5. An apparatus for manufacturing spheroidized beads inaccordance with claim 4 wherein said cathode is provided with a sourceof argon gas.
 6. An apparatus for manufacturing spheroidized beads inaccordance with claim 4 wherein said anodes are provided with a sourceof nitrogen gas.
 7. An apparatus for manufacturing spheroidized beads inaccordance with claim 1 wherein said plasma flame assembly provides aplasma flame and said plasma flame is supplied with an oxidizing gas. 8.An apparatus for manufacturing spheroidized beads in accordance withclaim 7 wherein said oxidizing gas comprises oxygen.
 9. An apparatus formanufacturing spheroidized beads in accordance with claim 1 wherein saidexhaust system includes means to allow release of gases and dustparticles from said walled chamber while maintaining a sufficiently highpositive pressure in said walled chamber.
 10. An apparatus formanufacturing spheroidized beads in accordance with claim 1 wherein saidwalled chamber has a diameter of at least 2 feet and a height of betweenabout 5 and about 30 feet.
 11. An apparatus for manufacturingspheroidized beads in accordance with claim 1 wherein said carbon arcplasma flame assembly and said walled chamber are provided with coolingjackets.